When you allow users to sign-up on yor seafile installation. They can enter whatever email they prefer. They can even add mailaddresses that dosen’t belong to the user since there is no verification when signing-up.
I suggest that if you set ENABLE_SIGNUP to True. And a user registrate an user account. There will be an verification mail sent to the user, asking to verify the account.
An option would be to have an setting in seahub_settings.py to enable it aswell. Like ENABLE_SIGNUP_VERIFICATION. But I would suggest to force this setting as it would secure most setups.